Evidence obtained in this laboratory indicates that an active transport mechanism may exist for oxygen into living cells. There are also indications that the mechanism can be disabled when exposed to ionizing radiation. It is planned to test for the existence of this mechanism through the use of agents which act on membranes and respiration. The role of the cell membrane (plasma or nuclear) as a site of damage leading to cell killing, for cells radiosensitized by oxygen, will also be examined. These studies will be conducted through utilization of the property that breaking survival curves are obtained when a thin layer of oxygenated cells are exposed to single 3 nsec duration pulses of ionizing radiation. These curves provide a method for determination of the amount of dissolved intracellular oxygen at the instant of pulse delivery. Both microorganisms and mammalian cells will be employed in the investigation. Another goal is to study the nature and kinetics of the early events induced in the living cell by high LET densely ionizing radiation. Investigations will be carried out using intense ultra high dose rate beams of sparsely ionizing electrons to irradiate radioresistant microorganisms. This will result in a large flux of secondary charged particles effectively similar to the situation achieved with high LET radiation. Initial experiments indicate that similar to the effect observed with high LET radiation, the OER can be reduced with such a high dose rate beam. Experiments will be performed using dual field emission sources capable of delivering 3 nsec duration pulses of radiation to a sample. The interpulse times are electronically controlled and variable. Through the use of this capability, it is hoped to determine the time scale of early events and its affect on OER, RBE and extrapolation number, which are the parameters affected by the LET of the radiation. The use of densely ionizing (high LET) radiation is being widely explored as a modality for the radiotherapy of cancer. These studies are expected to yield some understanding of the basic mechanisms involved in cell killing by this type of radiation, and thus have a bearing not only for fundamental radiobiology but also for radiotherapy.